Extension
An extension is a language feature that adds functionality to an existing type without modifying its original definition.
Kotlin in Action, 2E
An extension function is a Kotlin function defined outside a class that can be called with member-like syntax on values of an existing receiver type.
- How it is used
- Adapts existing Java and external APIs to idiomatic Kotlin member-like calls without rewriting their classes.
- How deep it goes
- Explains declaration, receivers, Java compilation, static dispatch, nullable receivers, and utility design; defers subclass syntax details.
- Examples
- Demonstrates String.lastChar, Collection.joinToString, User validation, nullable String checks, View/Button dispatch, and site-visit averages.
- What you can do after
- Apply it to add focused utilities and API adapters while avoiding assumptions about overriding or private-member access.
Functional Programming in Scala, Second Edition
An extension method is a method defined outside a type's original declaration that can be invoked with object-oriented syntax on values of that type.
- How it is used
- Adds object-oriented call syntax to functions defined separately from an algebraic data type.
- How deep it goes
- Explains method-versus-function tradeoffs, type specialization, receiver binding, compiler lookup, and companion-object placement.
- Examples
- Uses recursive `Tree` operations: generic `size` and integer-specific `firstPositive`.
- What you can do after
- Apply it via Scala extension syntax to add type-specific operations without editing the original type declaration.
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.NET in Action, Second Edition
An extension method is a static method that can be invoked with instance-method syntax on a type without modifying or inheriting from that type.
- How it is used
- Uses extension methods to add fluent behavior to existing types in a console application's command-line parsing workflow.
- How deep it goes
- Defines declaration syntax, invocation syntax, chaining, and visibility limits through focused C# examples.
- Examples
- Uses EveryOtherCharacter on string values and WithParsed or WithNotParsed on ParserResult objects.
- What you can do after
- Build it from scratch by defining a static extension class and invoking its method with instance syntax.
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Fabulous Adventures in Data Structures and Algorithms
An extension method is a statically declared method that can be invoked with instance-method syntax on a type without modifying that type.
- How it is used
- Uses static methods to extend interfaces and abstractions without enlarging their contracts or modifying their implementations.
- How deep it goes
- Demonstrates syntax, interface-design consequences, fluent composition, and concrete implementations, but does not develop compiler-resolution rules.
- Examples
- Uses immutable stacks, memoizing quadtrees, random distributions, histograms, sample sequences, and Euclid’s GCD algorithm.
- What you can do after
- Build extension-based APIs that preserve compact interfaces, support fluent calls, and reuse operations across related types.
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Pro ASP.NET Core 7, Tenth Edition
An extension method is a statically defined method that can be invoked with instance-method syntax on a type without modifying that type's definition.
- How it is used
- Adds application behavior to Microsoft, third-party, or otherwise unmodifiable types while retaining convenient instance-method syntax.
- How deep it goes
- Defines static classes, this parameters, interface targets, nullable handling, and yield-based filtering, but does not discuss compiler transformation internals.
- Examples
- Uses ShoppingCart, Product arrays, TotalPrices, FilterByPrice, controller actions, browser URLs, and displayed currency totals.
- What you can do after
- Build new operations for inaccessible classes, reuse them across interface implementations, and compose filtering with aggregation.
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Related concepts
- this pointerconfigured by · The this pointer is a language-provided reference to the current object instance in an instance method or related operation.
- receiveruses · A receiver is an entity that accepts a message or signal, or an object on which a method or command operates.
- functionkind of · A function is a defined operation that maps inputs to outputs and, when implemented as a callable program unit, may also produce effects.
- concatenationincludes · Concatenation is the operation of combining values or ordered sequences into a single composite, typically preserving their order.
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